1. Field of the Invention
The invention relates to premium synthetic lubricant base stocks derived from waxy Fischer-Tropsch hydrocarbons, their preparation and use. More particularly the invention relates to a high VI and low pour point synthetic lubricating oil base stock made by reacting H.sub.2 and CO in the presence of a Fischer-Tropsch catalyst to form waxy hydrocarbons boiling in the lubricating oil range, hydroisomerizing the waxy hydrocarbons having an initial boiling point in the range of 650-750.degree. F., dewaxing the hydroisomerate, removing light ends from the dewaxate and fractionating to recover a plurality of base stocks from the dewaxate.
2. Background of the Invention
Current trends in the design of automotive engines require higher quality crankcase and transmission lubricating oils with high VI's and low pour points. Processes for preparing lubricating oils of low pour point from petroleum derived feeds typically include atmospheric and/or vacuum distillation of a crude oil (and often deasphalting the heavy fraction), solvent extraction of the lube fraction to remove aromatic unsaturates and form a raffinate, hydrotreating the raffinate to remove heteroatom compounds and aromatics, followed by either solvent or catalytically dewaxing the hydrotreated raffinate to reduce the pour point of the oil. Some synthetic lubricating oils are based on a polymerization product of polyalphaolefins (PAO). These lubricating oils are expensive and can shrink seals. In the search for synthetic lubricating oils, attention has recently been focused on Fischer-Tropsch wax that has been synthesized by reacting H.sub.2 with CO.
Fischer-Tropsch wax is a term used to describe waxy hydrocarbons produced by a Fischer-Tropsch hydrocarbon synthesis processes in which a synthesis gas feed comprising a mixture of H.sub.2 and CO is contacted with a Fischer-Tropsch catalyst, so that the H.sub.2 and CO react under conditions effective to form hydrocarbons. U.S. Pat. No. 4,943,672 discloses a process for converting waxy Fischer-Tropsch hydrocarbons to a lube oil base stock having a high (viscosity index) VI and a low pour point, wherein the process comprises sequentially hydrotreating, hydroisomerizing, and solvent dewaxing. A preferred embodiment comprises sequentially (i) severely hydrotreating the wax to remove impurities and partially convert it, (ii) hydroisomerizing the hydrotreated wax with a noble metal on a fluorided alumina catalyst, (iii) hydrorefining the hydroisomerate, (iv) fractionating the hydroisomerate to recover a lube oil fraction, and (v) solvent dewaxing the lube oil fraction to produce the base stock. European patent publication EP 0 668 342 A1 suggests a processes for producing lubricating base oils by hydrogenating or hydrotreating and then hydroisomerizing a Fischer-Tropsch wax or waxy raffinate, followed by dewaxing, while EP 0 776 959 A2 recites hydroconverting Fischer-Tropsch hydrocarbons having a narrow boiling range, fractionating the hydroconversion effluent into heavy and light fractions and then dewaxing the heavy fraction to form a lubricating base oil having a VI of at least 150.